By the end of this lesson
- Define biodiversity.
- Distinguish species richness from evenness.
- Explain why biodiversity matters for ecosystem stability and human use.
BIG QUESTION
Does a field with more animals always have higher biodiversity?
A huge population of one species can mean low biodiversity; variety and evenness matter more than total number.
Biodiversity is variety
biodiversityThe variety of living organisms in an area, which can be described by species richness and evenness, and also includes genetic diversity. is the variety of living organisms in an area. At this level it is usually measured as the number of different species present and how many individuals belong to each.
Biologists also recognise genetic diversityThe variety of different alleles within a species or population. within a species and the variety of ecosystems in a region. These are wider meanings of the same idea.
Richness and evenness
species richnessThe number of different species present in an area, without counting how many individuals each has. is the number of different species. evennessHow similar the numbers of individuals of each species are in an area; a community dominated by one species has low evenness. describes how equally individuals are spread among those species.
Two meadows might each contain ten species. If one meadow is 95% grass of one species, it is less diverse than a meadow where the ten species are similarly common.
Why biodiversity matters
Ecosystems with higher biodiversity often have more feeding links, so the loss of one species may have a smaller effect on others. This tends to make them more resilient, although the effect depends on which species are lost.
Biodiversity provides food, medicines, materials and services such as pollination, so its loss can have economic as well as ecological consequences. Extension: diversity indices combine richness and evenness into one number.
Describing biodiversity
Use the precise term for each idea.
| Measure | What it describes | Example |
|---|---|---|
| Species richness | Number of different species present. | Meadow A has 12 plant species. |
| Evenness | How equally individuals are spread among species. | One dominant grass gives low evenness. |
| Genetic diversity (wider meaning) | Variety of alleles within a species. | Many varieties of wild wheat. |
| Population size | Number of one species (not a biodiversity measure alone). | 5000 daisies on a lawn. |
Language that earns precision
- biodiversity
- The variety of living organisms in an area.
- species richness
- The number of different species in an area.
- evenness
- How equally individuals are distributed among the species present.
- resilience
- The ability of an ecosystem to recover from disturbance.
Common misconceptions
Claim: More individuals means more biodiversity.
Correction: Biodiversity is about variety. Thousands of one species is low diversity.
Claim: Biodiversity only means animals.
Correction: It includes plants, fungi, microorganisms and all other living organisms.
Claim: High biodiversity guarantees an ecosystem cannot be damaged.
Correction: It often increases resilience, but losing key species or severe disturbance can still cause large changes.
Match the answer to the command word
Write “the variety of living organisms (or species) in an area”, not “the number of organisms”.
When comparing two areas, refer to both the number of species and how evenly they are represented.
Recall, apply, then justify
State what is meant by the term biodiversity.
Hint
Variety, not number.
Suggested marking points
- The variety of (different species of) living organisms in an area.
Two woodlands both contain 15 tree species. Explain why they may not have the same biodiversity.
Hint
Richness is the same; what else matters?
Suggested marking points
- The number of individuals of each species may differ.
- A woodland dominated by one species has lower evenness (and so lower biodiversity).
Explain why an ecosystem with high biodiversity may be more stable when one species declines.
Planning hint
Food webs, alternative food sources and functions.
What a strong design includes
- High biodiversity often means a more complex food web.
- Consumers may have alternative food sources.
- So the decline of one prey species affects predators less.
- Other species may carry out a similar role, such as pollination or decomposition.
- Population changes are buffered rather than spreading through the web.
- The effect depends on whether the declining species is a key species.